The proper way to copy a message to another file...?

[Steve Baker]

I have just started using gnus for mail and news (previously I used
Thunderbird.) There have been a few interesting "gotchas" but mostly
it's gone quite smoothly. However, I have run into one problem I have
been unable to figure out.

On a solaris 9 box (if that makes any difference), I use nnfolders as
my primary mail backend. I split out a few categories, but most of my
mail ends up in a single file, where I read it. Once I read it, I may
want to copy it to another file which is in the same nnfolders
directory. I use gnus-summary-save-in-mail to do this. The problem is
that the message is appended faithfully onto the end of the file,
including the X-Gnus-Article-Number header that was assigned to it
when it was originally received. Another message with the same number
in X-Gnus-Article-Number almost certainly will exist, and this leads
to two messages with the same number in the .nov file. The practical
effect is that when I go to the new file to look at the message, I see
it in the summary, but when I select it, I end up with the OTHER
message that has the same number (and whih does NOT appear in the
summary. 

How can I copy a message to another nnfolders file and have the
X-Gnus-Article-Number be correct for the new file? Am I missing some
small thing, or I am trying to about this completely wrong?

Thanks
-- 
Steve Baker <steve@tandika.com> | kiku wa ittoki no haji
                                | kikanu wa matsudai no haji